One of the major obstacles toward photon counting detector (PCD)-based clinical x-ray CT systems is the large count rates, because when operated under too intense x-rays, pulse pileup effects (PPEs) due to coincident photons distort the spectrum recorded by PCDs. In this paper we discuss a strategy using a hybrid detector, which consists of PCDs for the central part of the detector [which corresponds to a central small field-of-view (FOV) of the object] and energy integrating detectors (EIDs) for the peripheral part, to achieve the following three goals: 1) to minimize the PPEs; 2) to produce accurate spectral images for the small FOV; and 3) to provide conventional CT images for the entire FOV. The third goal requires a solution to exterior problem, because the central part of EID data is missing. The spectral data obtained by PCDs carry richer information than the intensity data obtained by EIDs; however, performing a simple weighted summation of counts from multi-energy windows of PCD would not produce realistic EID data, as the spectrum recorded by PCD could be skewed by spectral response effects (SREs) and PPEs. We propose a unique approach for the hybrid PCD/EID-CT system in this paper.
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